1. Field of Invention
The invention relates to a method for producing a foamed open-cell microporous polymer molded body comprising at least one thermoplastic polymer, and microporous foamed polymer molded bodies in the form of polymer particles of open-cell pore structure comprising at least one thermoplastic polymer.
2. Description of Related Art
Microporous polymer molded bodies find a variety of applications, in which they are used in a number of different forms. A wide area of application of microporous polymer molded bodies in particle form is the masterbatch technique, in which open-cell porous polymer particles are loaded with liquid additives or even solid additives that are soluble in liquids. Examples of such additives are flame retardants, antioxidants, antistatics and plasticizers. The loaded polymer particles are then mixed in the application as, for example, an additive masterbatch to a base polymer, by which means a homogeneous distribution of the additives in the base polymer can be achieved even at very low concentration. A further application involves porous polymer particles loaded with active substances, whereby active substances such as fragrances or pharmaceutical active ingredients such as drugs are introduced into the porous structure of the particles and released into the environment slowly and in a controlled manner in the application. The loading capacity of the porous polymer particles plays an important role in these applications. For example, loading with additive or active ingredient of up to 70 wt. % relative to the total weight of polymer and additive or active ingredient is sometimes required. Among the prerequisites for this is a high porosity of the polymer particles. The polymer particles must also possess sufficient stability, i.e., a compressibility that is as low as possible, otherwise, on storage of the particles loaded with additives or active substances in, for example, containers or sacks, the additives or active substances may be released from the particles.
Finally, porous polymer particles can also be used, on account of their large internal surface area, to absorb liquids, as for example in the separation of oils from water.
Microporous polymer molded bodies in the form of hollow-fiber membranes or flat membranes find varied application in the filtration of fluids, particularly in the area of ultrafiltration and microfiltration. In this case it is attempted to obtain high porosities, in the absence of which the attainable throughput through the membranes is too low. On the other hand the maximum porosity that can be set is often limited by the requirement that membranes used in processing and application have a certain minimum strength.
Various methods are known for production of microporous polymer molded bodies. DE 27 37 745 C2, for example, describes a method for producing microporous polymer molded bodies, based on a process involving thermally induced phase separation. In this method a homogeneous solution of a polymer component in a suitable solvent system is first produced at elevated temperatures. The polymer components and the solvent system form a binary system that in the liquid state of aggregation has a region where it is present as a homogeneous solution, and another region in which it possesses a miscibility gap. Cooling of such a system below the demixing temperature results in phase separation and finally in the formation of a porous polymer structure. Methods of this type for membrane production are described also in, for example, DE-A-32 05 289 and EP-A-0 133 882.
Another method for producing porous polymer particles is described in WO 98/55540, in which a polyolefin polymer is dissolved in a solvent and the solution dispersed, at a temperature above the crystallization temperature of the polyolefin, in a non-solvent for the polyolefin, with the formation of a multiphase system. Porous polyolefin particles are obtained on cooling of the dispersion.
These known methods allow production of polymer molded bodies of high porosity and open-pored structure as well as high loading capacity. However, the above methods for their production have the disadvantage that the required use of solvents necessitates costly extraction and/or drying processes for removal of these solvents. Despite this costly extraction and/or drying, complete removal of the solvent from the polymer molded bodies obtained is usually not achieved, resulting in restrictions on the use of such polymer molded bodies in the areas of, e.g., medicine or food technology, or even in the electrical industry.
Another method proposed for the production of porous polymer molded bodies involves releasing the pressure on a pressurized melt consisting of a thermoplastic polymer containing a volatile blowing agent. U.S. Pat. No. 5,160,674, for example, describes a method for producing foamed materials from semicrystalline polymers, in which a pressurized melt of the polymer used is saturated with a gas and shaped, also under pressure; the reduction of pressure after removal from the die then causes foaming of the polymer material. Foamed materials produced by the method of U.S. Pat. No. 5,160,674 show a homogeneous porous structure, wherein the pores or cells are however closed. Closed-cell materials of this type cannot be loaded with additives or active substances, however, and are also unsuitable as membranes because they allow no throughput, or at best very low throughput, of the fluid to be filtered.
DE-A 44 37 860 describes a method for production of sheet-like microcellular foams from amorphous thermoplastic polymers such as polystyrene, whereby a thermoplastic polymer melt is impregnated with a volatile blowing agent in a first extrusion zone and the melt containing the blowing agent is then cooled in a second extrusion zone by at least 40° C. to a temperature lying at least 30° C. above the glass transition temperature of the polymer containing the blowing agent. On releasing the pressure on the melt to normal pressure and cooling it to room temperature, the melt expands and solidifies to a foam sheet.
WO 00/26006 describes a method for producing microcellular foams from polymers or polymer mixtures for forming of molded bodies such as hollow-fiber or flat membranes. In a first extrusion zone a polymer melt is loaded with a compressed gas under the action of a shearing and/or kneading means, and in a second extrusion zone the solubility of the gas in the gas-loaded melt, and therefore foaming, are improved by increasing the pressure. The foamed molded bodies obtained as a consequence of pressure release after extrusion through a die may have either an open-cell or closed-cell structure depending on the setting of the process conditions, the mean cell size, according to the disclosed example, lying in the range of approx. 10 μm. The foams obtained by the method described in WO 00/26006 have high porosity in a range higher than 90 vol. %. In many applications, however, foamed structures of this type cannot be used on account of their poor mechanical stability.
WO 99/38604 discloses foamed porous membranes made from thermoplastic polymers and a method for their production. The membranes described in WO 99/38604 have a mean pore diameter of between 0.05 and 30 μm, a porosity of at least 75 vol. % and a proportion of open cells of at least 80%. They are produced by a method in which a polymer melt comprising at least one polymer is conveyed through an extrusion device under pressure and loaded with a cell former in an injection stage. The pressure in this part of the extrusion device is set to at least 150 bar and the temperature, which is above the glass transition or melting temperature, is so chosen that correct and smooth functioning of the extrusion device is guaranteed. In a downstream mixing step, a single-phase melt is produced from the at least one polymer and the cell former, the temperature of the melt in the mixing step being reduced below the working temperature in the previous part of the extrusion device and/or the pressure raised. The single-phase melt is extruded through a die for shaping of the membrane, during which the cell former foams the polymer melt as a consequence of the resulting fall in pressure. To attain the desired proportion of open cells, the cell formers consist according to WO 99/38604 of at least two components, which are gases and/or low-boiling liquids that must have different speeds of diffusion relative to the polymer melt. The high proportion of open cells is ascribed, according to WO 99/38604, to the opening of cells that were initially closed, apparently because the blowing agent with the lower diffusion speed gives rise to a high internal cell pressure resulting in rupture of the cell walls, which are very thin on account of the high porosity also required. It is clear from this that the method of WO 99/38604 can be used at best to only a limited extent for production of foamed structures of low porosity. Limitations are also placed on the applicability of the method by the requirement that the cell former must consist of at least two components of different diffusion speeds relative to the polymer melt.